Compressing the light beyond the diffraction limit is one of the most fundamental problems of modern photonics and plasmonics. Efficient coupling between diffraction-limited and sub-diffraction scales will strongly benefit the areas of near-field sensing, nm-scale optical control, single-molecule spectroscopy, high-energy focusing and compact optoelectronics. While light emission by atoms, molecules, quantum wells, quantum dots and other quantum objects occurs from nm-sized regions, light propagation takes place on μm-wide (wavelength) scales. Such a huge scale difference introduces fundamental limitations on (i) the size of waveguiding structures and (ii) efficiency of coupling between nano- and micro-domains. These limitations, in turn, restrict the resolution and sensitivity of near-field microscopes, prevent fabrication of ultra-compact all-optical processing circuits, integrated optoelectronic devices and other photonic systems. However, despite the ever-increasing number of opportunities offered by modern technology, straightforward reduction of size of conventional dielectric waveguides is not possible since the onset of diffraction will lead to either cut-off of waveguide modes or to their leakage into the dielectric surrounding.
A number of approaches to confine the optical signals beyond the diffraction limit has been suggested. These techniques can be separated into two groups. The methods of the first group rely on the excitation of special kind of electromagnetic waves coupled to the collective oscillation of electron plasma, known as surface plasmon polaritons (SPPs). The second group utilizes exponentially decaying (evanescent) fields in tapered waveguides to transmit the information beyond the diffraction limit. However, both these approaches have their limitations. Namely, the dispersion properties of SPPs are substantially different from the dispersion of volume waveguide modes. Therefore, excitation of SPPs requires fabrication of special kinds of coupling structures which can introduce substantial energy loss.
Exponential decay associated with evanescent waves, on the other hand, severely deteriorates the signal-to-noise ratio, limiting the minimum power required to operate the systems and maximum separation between the transmitter and receiver in these structures. Although some approaches involving resonant excitation of surface modes in negative-index or negative-permittivity systems have been promised to restore the evanescent waves, all practical realizations of these structures are so far limited to the near-field proximity of the sub-wavelength source.
Hence, current waveguide technology is unable to effectively confine propagating electromagnetic waves to the regions substantially smaller than half of the optical wavelength (typically, about 300 nm). State of the art plasmonic and plasmon enhanced waveguides are capable of transferring energy to subwavelength scales, but due to the incompatibility of the mode structure with free-space radiation, typically, only 0.5% of initial energy reaches the tip of a fiber in modern near-field couplers.
In Phys. Rev. B 71, 201101 (R) (May 2005), Podolskiy et al. have proposed a nonmagnetic, nonresonant approach to build left-handed media (LHM) using a planar waveguide with anisotropic dielectric core. (Left-handed media, also called negative index media (NIM) or negative phase velocity materials (NPVM), are typically characterized by both negative dielectric permittivity and negative magnetic permeability.) The combination of strong anisotropy of the dielectric constant and planar waveguide geometry yields a negative phase velocity. In ArXiv:physics/0506196 (June 2005), Podolskiy et al. discuss details of the proposed LHM. One material mentioned is a layered stack of alternating dielectric and polar or plasmonic materials. It is generally suggested that a related positive index material made of Ag—Si layers potentially could be used to concentrate propagating modes in subwavelength areas. (ArXiv:physics/0506196. Also US Patent Application 2006/0257090.) There is not, however, any specific discussion or suggestion that the concentration would be in more than one dimension.
Thus, there is a need in the art of optical materials for waveguide technology capable of confining propagating electromagnetic waves to the regions smaller than half of the optical wavelength to permit effective coupling between diffraction-limited and sub-diffraction scales.